In lithography, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography, which are however rather costly.
A potentially less costly road to smaller features that has gained increasing interest is so-called imprint lithography, which generally involves the use of a “stamp” (often referred to as an imprint template or an imprint lithography template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
Imprint lithography involves the patterning of an imprintable medium on a surface of a substrate to be patterned. The patterning may involve bringing together a patterned surface of an imprint template and a layer of imprintable medium (e.g., moving the imprint template toward the imprintable medium, or moving the imprintable medium toward the imprint template, or both) such that the imprintable medium flows into recesses in the patterned surface and is pushed aside by protrusions on the patterned surface. The recesses define pattern features of the patterned surface of the imprint template. Typically, the imprintable medium is flowable when the patterned surface and the imprintable medium are brought together. Following patterning of the imprintable medium, the imprintable medium is suitably brought into a non-flowable or frozen state (i.e. a fixed state) and the patterned surface of the imprint template and the patterned imprintable medium are separated. The substrate and patterned imprintable medium are then typically processed further in order to pattern or further pattern the substrate. The imprintable medium is typically provided in the form of droplets on the surface of a substrate to be patterned, but may alternatively be provided using spin coating or the like.
In optical lithography, where a beam of radiation is passed through or reflected off a patterning device (such as a mask or the like), the radiation beam will be patterned according to the pattern provided by the patterning device. The patterned radiation beam is then projected onto a substrate in order to provide one or more patterns on that substrate. Due to non-uniformities in the patterning device, or other elements of an optical lithography apparatus, the pattern projected onto the substrate may not be as intended. For example, heating of the patterning device or a projection lens used to project the patterned radiation beam onto the substrate may cause the patterning device or lens to deform, which may in turn introduce non-uniformities into the patterned radiation beam that is projected onto the substrate. Non-uniformities in the pattern provided on the substrate may comprise, for example, an increase or a decrease in a dimension of a pattern of the radiation beam and thus in a feature provided on the substrate. It is desirable to ensure that the pattern features are provided on the substrate in a uniform and consistent manner across the substrate.
In order to overcome the problem of non-uniformities in optical lithography, the radiation dose provided by one or more parts of the radiation beam may be controlled in order to change the dose of radiation that is provided on different areas of the substrate. For instance, the dose of radiation may be decreased in areas where it is known that non-uniformities in the lithographic apparatus (or during subsequent processing) would otherwise result in pattern features being applied to the substrate with dimensions that are larger than intended. Similarly, the dose of radiation may be increased if it is known that the non-uniformities in the lithographic apparatus will result in pattern features having dimensions smaller than intended.